Since Discourse runs in Docker, why not develop there? If you have Docker installed, you should be able to run Discourse directly from your source directory using a Discourse development container.

Step-by-step

It should be as easy as (from your source root):

./bin/docker/boot_dev --init
    # wait while:
    #   - dependencies are installed,
    #   - the database is migrated, and
    #   - an admin user is created (you'll need to interact with this)
./bin/docker/rails s

... then open a browser on http://localhost:3000 and voila!, you should see Discourse.

When you're done, you can kill the Docker container with:

./bin/docker/shutdown_dev

Note that data is persisted between invocations of the container in your source root tmp/postgres directory.

Caveats

There seems to be an issue with the ember-data-source gem installed by default (2.3.0.beta.5). It's missing its dist directory. I've worked around this by acquiring that commit, building the distribution locally, and patching it into /usr/local/lib/ruby/gems/2.3.0/gems/ember-data-source-2.3.0.beta.5 by hand. I believe later versions of the gem fix this, but the very next version (2.3.0 proper) bumps the ember-source dependency up to 2.0, which Discourse isn't using yet.

You can get boot_dev to patch for you by passing --patch local/path/to/ember-data-source/dist on the command-line. You should only have to do this once (like --init).

Other Notes

Where is the container image/Dockerfile defined?

The Dockerfile comes from discourse/discourse_docker on GitHub, in particular image/discourse_dev.